Many backrests types are known in the art. Most backrests are attached to a frame and do not allow any adjustment possibilities. Adjustable backrests are also known. For instance, document US 2006/0091706, Christofferson et al. “Seat assembly for wheelchair”, describes a typical seat assembly that can be mounted on various wheelchair bases. The seat assembly enables the seat width and backrest width to be adjusted independently of each other. The backrest has a plurality of open slots, the number and orientation of which contributes to depth, width, and height adjustment of the backrest as well as angular adjustment of lateral supports to permit the backrest to conform to the anatomical curves of a user. A first set of slots is provided for height adjustment of the thoracic support. Further slots are provided for lateral adjustment of the thoracic support. Threaded fasteners provided in adjustment holes are provided for height adjustment of the backrest with respect to a main member. In the disclosed embodiment, all adjustments are made independently, involving time consuming manipulations. Moreover, once a first adjustment has been made with a first member, further adjustments of this first member may be required after another member is adjusted. Finally, in this known wheelchair, the backrest comprises a central part and two lateral parts which are rigid. Therefore, it is not possible to deform these parts so as to adapt the shape of the backrest to the anatomical curves of a user.